In a power circuit, a semiconductor switching element is used, and reductions in surge voltage during switching and in switching loss are important subjects. In the related art, methods are proposed in which gate voltage during switching of the semiconductor switching element is actively controlled so that switching loss is reduced while the surge voltage is maintained at a low level. In these methods, an element voltage, an element current, and a gate voltage of the switching element to be driven are detected, and a logic process is applied by a single detection signal or combination of detection signals, to control the gate voltage. The gate voltage is controlled by changing the voltage of the gate circuit or changing the gate resistance. Japanese Patent No. 3141613 and JP 2001-197724 A exemplify semiconductor switching elements.
However, when the element current is to be detected, flux-concentration type magnetic current sensors have problems in that they are costly and the sensor size is large. Hall magnetic sensors and single magnetic resistance elements have inferior precision and are also costly. In addition, as the current value to be used for the control, a value immediately before the switching is necessary, and thus high speed detection is required. However, it is difficult to achieve both precision and high speed detection at the same time.
In addition, when the gate voltage is to be detected, there also are problems in that the gate voltage shows complex behaviors because feedback is applied from a parasitic capacitance of the driving element during switching, and that switching noise from other circuits connected to the same ground line tends to mix because the power supply is approximately 15V. In addition, because a change point of the gate voltage becomes a branch point in the circuit operation, a circuit operation monitor signal is detected by a differential the voltage. However, because the voltage differential signal has a small value, a large control error is caused by other noise, and thus the voltage differential signal is unsuited as a control signal during switching.
On the other hand, although detection of the element voltage is relatively easy because the element voltage has a large value, the controlling point of the gate voltage must be optimized. In the above-described Patent References 1 and 2 also, the element voltage is used. However, in Patent Reference 1, there is no disclosure of the detection point and, in Patent Reference 2, a point in which the element voltage is low is set as the detection point and there is a problem in that variation of elements cannot be handled.